Television system



, 1935. R. CAMPBELL TELEVISION SYSTEM March 26 Filed 001:. 29, 1932 Patented 26, 1935 V V TELEVISION SYSTEM Richard L. Campbell, Oaklyn, N. 3., assignor to Radio Corp. of America, New York, N. E, a corporation of Delaware Application October 29, 1932, SerlalNo. 640,169

9 Claims. (Cl. 178-6) My invention relates to improvements in tele- In Fig. 1, a transmitting tube 10 is provided vision systems. with an electrode 12, having the usual photosensi- In television systems embodying a cathode ray tive surface 14 comprised of a great number of tube at the transmitting station, and in which minute metallic elements, such as silver globules,

UNITED STATES PATENT OFFICE the tube comprises aphotosensitive mosaic screen each insulated from the electrode 12 and from '5 on which an image of the object is projected, and each other, and each provided with a coating of which is scanned by a cathode ray, it has been photosensitive material. In operation, an image found to be advantageous to project the image of the object, shown as being inthe form of a on the same side of the screen which is scanned moving picture film 16, is projected upon the by the ray. When this is done, the axis of the photosensitive surface by a suitable optical sys- 10 optical system for projecting the image is made tem 1'7, the axis 18 of the optical system being perperpendicular to the screen, but the axis of the pendicular to the surface or screen 14. gun for developing and directing the ray of elec- A gun 20 of any suitable construction operates trons is, of necessity, disposed at an acute angle to develop and direct-a ray 24 of electrons at the to the screen. on account of this arrangement, photosensitive surface 14, the axis 22 of the gun I; the outline of the area scanned by the ray has a being disposed at an acute angle, for example. keystone shape rather than the desired rectangu- 60, to the photosensitive surface 14. lar shape. This distortion of the scanned pattern For the purpose of causing the ray 24 to scan is manifested by a. substantial and corresponding the surface 14, a saw-tooth current wave, at a distortion of the reproduced image at the receivi frequency of 24 cycles, is caused to flow through 20 ing station, the reproduced image being also the coils 28 wound on poles 28a. Also, for this purshape of a keystone, with the vertical lines inpose, a saw-tooth current wave, at a frequency clined to each other. i of 4320 cycles, is caused to fiow through deflect- With the. foregoing in mind, it is one of the ing coils 32. The coils 28' deflect the ray horiobjects of my invention to provide an improved zontally and the coils 32 deflect the ray vertically. 25

television system of the character referred to The ray 24, therefore, is caused to scan the surwherein the keystone distortion is corrected. face 14 twenty-four times a second, each scan- Other objects and advantages will hereinafter ning being along 180 horizontal lines. appear. In operation, the projection of the image on 80 In accordance with my invention, the cathode the photosensitive surface 14 causes discrete elecray is caused to pass through a magnetic field trostatic charges to be stored up over the surof constant strength, the distribution of flux face, each elemental charge being proportionate density and the direction of the lines of force of to the light intensity at the elemental area. As this field being such as to cause the ray to scan the ray 24 scans the surface 14, these charges are the screen structure over a substantially rectanguneutralized to develop picture signals. 35 lar area. I In the system just described, the ray 24 would More particularly, in accordance with my inscan the surface 14 over an area ABCD, as shown vention, magnet poles are disposed about the axis in Fig. 3, and as further illustrated in Fig. 1. of the ray developing means to establish a fixed That is, the scanning pattern would be the shape 40 magnetic field, the arrangement being such that of a keystone rather than the desired rectangle. 40 the keystone distortion which would otherwise This would cause, at the receiving station, the occur is corrected for. production of a distorted image on the fluorescent ,My invention resides in the improved method screen of the usual cathode ray tube, the vertical of operation and system of the character herelines of the. reproduced image being so inclined inafter described and claimed. to each other that the entire image would have 45 For the purpose of illustrating my invention an a generally keystone configuration. embodiment thereof is shown in the drawing, For the purpose of compensating .or correctwherein ing for the keystone distortion referred to, I Figure 1 is a perspective view, partly diagrampropose to provide a fixed magnetic field through 6 matic, of television transmitting apparatus emwhich the ray 24 passes during the scanning v bodying my invention; action. The distribution of the flux density and Fig. 2 is a simplified sectional view, the section the direction of the lines of force in this field being taken on the line 2-2 in Fig. 1; and are such that the ray is caused to scan a re- Figs. 3 and 4 are diagrammatic views illustraversed keystone area EFGH in a plane perpendic- 66. tive of the principle of operation. ular to the axis 22. The ray, therefore, is actually caused to scan the surface 14 over the rectangular area JKLM.

The fixed magnetic field is provided by a U- shaped magnet 34 disposed with the poles 36 and 38 of opposite polarity directed at the screen 14 and disposed, respectively, on opposite sides of the axis 22. The poles 36 and 88 extend somewhere beyond the deflecting poles, so that the ray 24 passes first through the constantly changing deflecting field and then through the fixed keystone-correcting field.

The common axis 40 of the poles 28a is perpendicular to and intersects the axis 22. The axis 42 of the magnet 34 is substantially parallel to the axis 22 and is in the plane determined by the intersecting axes 22 and 40.

The poles 36 and 38sare parallel to the axis 22, and are disposed in a plane which is perpendicular to the plane determined by the intersecting axes 22 and 40.

The particular shape, strength, and exact location of the magnet 34' is determined by the trial-and-error method for each particular apparatus.

Obviously, an electromagnet may be substituted for the permanent magnet 34, to serve the same purpose as the latter. This provides the advantage of easier adjustment.

In some cases, it has been found that the area JKLM over which the ray 24 scans the surface 14 has distorted comers, as illustrated by the dash lines in Fig. 4. These distorted corners can be corrected or trimmed up by the use of auxiliary bar magnets 44 placed on opposite sides of the axis 22 in some such manner as is shown in Figs. 1 and 4. It will be understood that the exact position, strength and dimensions of these auxiliary magnets can best be determined, for any particular apparatus, by the trial-and-error method.

It will be understood that modifications are possible without departing from the spirit of my invention or the scope of the' claims.

I claim as my invention:

1. In apparatus of the character described, screen structure, means for developing a scanning ray and directing the same at said structure, the axis of said ray-developing means being inclined at an acute angle to said structure, and means for causing the ray to scan a keystone area cver a plane perpendicular to said axis whereby the ray is caused to scan the surface of said structure over a substantially rectangular area; said second-named means comprising means for deflecting said ray, and means for producing a magnetic field of varying strength which causes a deflection less than normal at one end of the screen and equal to or greater than normalat the other end of the screen.

2. In apparatus of the character described, screen structure, means for developing a scanning ray and directing the same at said structure, the axis of said ray-developing means being inclined at an acute angle to said structure, means for deflecting said ray to cause the same to scan said structure, and means for creating a fixed magnetic field through which said ray passes during the scanning action to cause the ray to scan a keystone area over a plane perpendicular to said axis whereby the ray is caused to, scan the surface of said structure over a substantially rectangular area, said last-named means comprising magnet poles of opposite polarity disposed respectively on opposite sides of the'axis of said ray-developing means. 7

3. In apparatus of the character described, screen structure, means for developing a scanning ray and directing the same at said structure, the axis of said ray-developing means being inclined at an acute angle to said structure, means for deflecting said my to cause the same to scan said structure, and means for creating a fixed magnetic field through which said ray passes during the scanning action to cause the ray to scan a keystone area over a plane perpendicular to said axis whereby the ray is caused to scan the surface of said. structure over a substantially rectangular area, said last-named means comprising a U-shape magnet disposed with its poles on opposite sides of the axis of said ray-developing means.

4. In apparatus of the character described, screen structure, means for developing a scanning ray and directing the same at said structure, the axis of said ray-developing means being inclined at an acute angle to said structure, means for deflecting said ray to cause the same to scan said structure, and means for creating a fixed magnetic field through which said ray passes during the scanning action to cause the relay to scan a keystone area over a plane perpendicular to said axis whereby the ray is caused to scan the surface of said structure over a substantially rectangular area, said last-named means comprising a U-shape magnet disposed with its axis in a plane with the axis of said ray-developing means and with its poles directed at said structure'and disposed in a plane substantially perpendicular to said first-mentioned plane.

5. In apparatus of the character described, screen structure, means for developing a scanning ray and directing the same at said structure, the axis of said ray-developing means being inclined at an acute angle to said structure, and means for causing the ray to scan a keystone area over a plane perpendicular to said axis whereby the ray is caused to scan the surface of said structure over a substantially rectangular area; said second-named means comprising magnet poles disposed'on opposite sides of said axis and having a common axis perpendicular to and intersecting said first-named axis, and magnet poles of opposite polarity disposed respectively on opposite sides of and substantially parallel to the plane determined by said intersecting axes.

6. In combination, an evacuated container in which is mounted screen structure as well as an electron gun for developing a stream of electrons and directing it toward said structure, the screen structure intersecting the path of the electron stream at an angle, means for deflecting the electron stream to cause it to scan the screen structure and means for creating a. magnetic field through which said stream passes during the scanning operation, said last named means comprising magnet poles of opposite polarity disposed, respectively, ,at opposite sides of the electron path and auxiliary magnet poles disposed, respectively, at opposite sides of said path and on the screen side of the deflecting means whereby the tendency of the stream to scan a keystone shape area on the screen may be compensated.

'7. The invention set forth in claim 6 characterized in that the surface of the screen on which the electron stream impinges is photo-sensitive.

ing inclined to the axis of the ray, means for deflecting the cathode ray to cause it to repeatedly scan the screen element and auxiliary means for supplying magnetic fields to influence the cathode ray independently of the deflecting means whereby the boundaries of the scanned area may be corrected.

' RICHARD L. CAMPBEIL. 

